Molecular Kinetic Study of "chain Shuttling" Olefin Copolymerization

Statistical olefin block copolymers (OBCs) with "hard" and "soft" linear low-density polyethylene (LLDPE) blocks can be synthesized by tandem catalysis under "coordinative chain transfer polymerization" (CCTP) conditions. This process, disclosed in 2006 and commonly referred to as "chain shuttling copolymerization" (CSCP), is now exploited commercially by Dow Chemical, to produce thermoplastic elastomers with the Infuse trade name. Whereas the general kinetic principles of CSCP as well as the fundamental physical properties of the products are rather well-understood, the details are still poorly defined, to the point that even average block numbers and lengths of commercial Infuse grades are not available in the public domain. In this paper, we report the results of a molecular kinetic investigation in which high throughput experimentation tools and methods were employed to unravel the microstructure and architecture of these materials. The problem was factored in two parts. First, each of the two catalysts in the original Dow Chemical formulation was studied individually in ethene/1-hexene CCTP. Next, the two catalysts together were used in CSCP experiments under otherwise identical reaction conditions. The robust database thus obtained enabled us to disambiguate the interpretation of the results, and sort out system behavior as a function of the relevant variables. Plausibly, the process turned out to be governed by the relative probabilities of "self-shuttling" versus "cross-shuttling" (that is, of exchanging blocks of the same or different type). In particular, the synthesis of OBCs with long hard blocks and an excess of soft blocks, which are those featuring the most desirable application properties, requires a moderate chain shuttling rate and an excess of the catalyst with the higher comonomer incorporation ability; as a result, at practical average molecular weight values, these products are characterized by a pronounced interchain disuniformity, with an abundant fraction of chains undergoing exclusively "self-shuttling" at the aforementioned catalyst, and therefore consisting of just one soft block.